Arizona Bureau of Geology and Mineral Technology Summer 1986 Volcanic by Stephen J. Reynolds, John W. Welty, andJon E. Spencer Arizona Bureau of Geology and Mineral Technology Although Arizona lacks active volcanoes, it was the site of massive outpourings of lava and volcanic ash during the recent and more distant geologic past. Volcanism not only formed the oldest rocks in Arizona 1.8 bi11ion years (b.y.) ago, but also constructed the State's highest mountains, the San Francisco Peaks, within the last 3 million years (m.y.). Volcanic rocks are widespread throughout the State and dominate much of its scenery (Figure 1). This article summarizes the Figure 1. Aerialphotograph ofS P Craterandbasaltflow In Ihe San Francisco volcanic volcanic history of Arizona and briefly describes the importance of field, north of Flagstaff The eruption that formed Ihis cinder cone and lava flow volcanic rocks in the State. occurred relatively recently, about 70,000 years ago. Photo by David D. Nations. A second episode of Precambrian volcanism occurred about 1.1 Volcanic Episodes to 1.2 b,y. ago, but was moreareally restricted and less voluminous than the previous episode. This volcanism is evident in basalt flows and Six major episodes of volcanism are represented within the rhyolite tuffs (compacted volcanic ash) interbedded with sedimentary geologic record of Arizona. These episodes 10cal1y, but unequal1y, rocks ofthe late Precambrian Apache Group nearGlobe and the Unkar affected al1 three of Arizona's geologic-physiographic provinces: Colo­ Group of the Grand Canyon (Figure 2). These rocks accumulated in rado Plateau, Transition Zone, and Basin and Range Province (Peirce, basins within the stable continental interior and were later injected with 1984). Volcanic activity within the last 10 m.y., for example, was intense horizontal sheetlike intrusions, or si11s, of diabase, a dark·colored along the boundarybetween the Colorado Plateau andTransition Zone, igneous rock that forms by crystallization of basaltic magma at depth. butwas less common in other parts ofthe State. Because each volcanic These diabase sills are well displayed in the walls of the Salt River episode was localized, mineral and energy resources related to Canyon northeast of Globe. The presence of basalt flows, tuff, and volcanism are also unequal1y distributed. This unequal distribution has, diabase sills within a continental basin suggests that volcanism and in turn, dramaticallyaffected the settlementofArizona and continues to sedimentation accompanied a period of minor continental rifting. exert some influence on the population distribution. After this second episode, volcanism was evidently absent in The earliest volcanic episode in Arizona occurred during the Arizona for almost 1 by, or more than half of the State's entire geologic Precambrian Era between 1.8 and 1.6 b.y. ago and formed much ofthe history. During this prolonged volcanic quiescence, which includes all continental crustthat underlies the region today. Volcanic rocks formed of the Paleozoic Era (570 to 245 m.y. ago), Arizona was a relatively during this time are most widely exposed in the Transition Zone and stable, topographically subdued region that was intermittently flooded parts of the Basin and Range Province (Figure 2). Volcanism probably by shallow seas. This interval of relative stability was interrupted in the began when the oceanic crustwas pulled apart, creating a rift in the sea early Mesozoic, when volcanic ash and detritus were blown by winds floor from which basalt was erupted. This process of sea-floor and washed by streams into northern Arizona from distant volcanoes to spreading is occurring today along the mid-oceanic ridges circling the the south and west, probably in northern Mexico or southern California Earth. Large volcanic islands, some similarto Hawaii and others similar (Peirce, 1986). The volcanic ash and debris were deposited in the to the Aleutian Islands in Alaska, were built upon the basaltic sea floor. Upper Triassic Chinle Formation and became altered to impart the The central volcanoes erupted basalt and more silica-rich lavas and variegated colors of the Painted Desert and Petrified Forest. ByJurassic were surrounded by aprons of sedimentary debris derived from time (190 m.y. ago), volcanism became widespread in a northwest­ explosive eruptions of volcanic ash and from erosion of the volcanoes. trending volcanic belt that crossed southern Arizona from Douglas to Heatfrom the central volcanic conduits caused water to circulate within Parker (Figure 3). Volcanoes within this belt erupted regionally and around the volcanoes, which led to deposition of mineral deposits extensive rhyolitic tuffs and more localized rhyolitic lavas. The volcanic such as the famous copper-silver-gold ores of Jerome. Col1isions landscape was locally overrun by sand dunes, believed to have between converging volcanic arcs deformed and metamorphosed the migrated southward from the great deserts that existed during volcanic and associated sedimentary rocks, locally converting them deposition of the Lower Jurassic Navajo Sandstone of the Colorado into schist (Anderson, 1986). During and after these collisions, large Plateau. The resulting interbedded Jurassic volcanic rocks and sand­ masses of granitic magma intruded into and helped stabilize the stones are present in the Santa Rita and Baboquivari Mountains of continental crust of the region. southern Arizona and in several mountain ranges near Parker. ---'l'~7-_l_-7::~~j'. --f---r o q I I i 0 \ i \_000 I I I I I 1_,,0 I : I MI a 50 I KM ot-r'-r-f--C5:!::0~ DISTRIBUTION OF PRECAMBRIAN VOLCANIC AND METAVOLCANIC ROCKS IN ARIZONA DISTRIBUTION OF JURASSIC (200-150 m.y.) VOLCANIC ROCKS Figure 2. Outcrops ofPrecambrian and metamorphosed uolcanic rocks formed during Figure 3. Outcrops ofJurassic uolcanic rocks occurin a northwest-trending belt across the first uolcanic episode (1.6-1.8 b.y. ago) are shown in black. Deformation and southern Arizona. metamorphism underhigh temperatures andpressures hauegenerally conuened these uolcanic rocks into schist or gneiss. Outcrops of rocks formed during the second episode (1.1-1.2 b.y. ago) are too small to show indiuidually. but occur in the Unkar granitic masses in the Santa Catalina Mountains, Baboquivari Moun­ Group in the Grand Canyon area (GC) and in the Apache Group near Globe (AG). tains, and southwestern Arizona. There was a pronounced lull in volcanic activity during the early In Late Jurassic time (145 m.y. ago), volcanism moved westward Tertiary from 50 to 35 m.y. ago (Shafiqullah and others, 1980). This out of Arizona and into coastal California and Baja California, where a interval was followed by a middle Tertiary episode of intense volcanism volcanic arc formed along the continental margin, much like the that covered much of the Basin and Range Province with regional present-day Andes of South America. This volcanism occurred above ash-flow tuffs and more areally restricted rhyolite, andesite, and basalt an east-dipping subduction zone, where a plate of oceanic crust in the flows. During this episode, a single catastrophic eruption west of Pacific Ocean descended into the mantle beneath coastal California Kingman deposited a layer of ash-flow tuff over an area of 35,000 km2 and Mexico. Volcanism continued along the coast during most of the between Peach Springs, Arizona and Barstow, California (Glazner and Cretaceous, whereas southern Arizona was largely free of volcanoes others, 1986). Volcanism swept westward across Arizona between 35 and was locally inundated by shallow seas from the east. and 15 m.y. ago, as the dip of the subducted oceanic plate decreased Arizona's fourth volcanic episode occurred during Late Cretaceous beneath Arizona (Coney and Reynolds, 1977). As a result, middle to early Tertiary time (85 to 45 m.y. ago), when volcanism and Tertiary volcanic rocks are mostly 25 to 35 m.y. old in southeastern associated igneous activity migrated eastward back into Arizona and Arizona, but 15 to 25 m.y. old in western Arizona. These middle Tertiary adjacent New Mexico. This eastward sweep of magmatism was due to a volcanic rocks are widespread throughout the Basin and Range decrease in the dip orangle ofthe subducted oceanic plate beneath the Province (Figure 5) and provide the backdrop for some of the region's region (Coney and Reynolds, 1977). Volcanic rocks ofthis age are most most scenic areas, including the Chiricahua, Galiuro, Superstition, and widely preserved in southeastern Arizona (Figure 4), but originally Hieroglyphic Mountains, Picacho Peak, the Kofa National Wildlife covered parts of western Arizona, where they have been removed by Refuge, Organ Pipe National Monument, and Apache Leap near erosion. This volcanic episode included construction of andesitic Superior. Middle Tertiary volcanic rocks are present, but much less stratovolcanoes, as well as catastrophic caldera collapse that accom­ common, in the Transition Zone and Colorado Plateau. Middle Tertiary panied eruption of large volumes of volcanic ash. Caldera collapse volcanism was locally accompanied by deposition of precious- and locally resulted in the deposition of breccias that contain house-sized base-metal veins from hot fluids that circulated near the volcanic blocks of limestone, granite, and other rocks derived from the walls or centers and along major fault zones. deeper levels of the caldera. Remnants of such calderas are probably The sixth and last major episode ofvolcanism in Arizona occurred present in the Silver Bell, Tucson, and Santa Rita Mountains near during the late Cenozoic since 15 m.y. ago. It coincided with faulting Tucson (Lipman and Sawyer, 1985). During this episode ofvolcanism, that formed many of the deep basins in the Basin and Range Province large porphyry copperdeposits formed in the cooling magma chambers and Transition Zone. In contrast to the previous episode, volcanism underneath the volcanoes. These copper deposits, like the associated largely consisted of basalt flows that are most widely exposed in the volcanic rocks, are most cornman in southeasternArizona, where they Transition Zone and along the southwestern edge of the Colorado escaped complete removal by erosion.